How to convert a NSString to long double? - ios

Question

I am dealing with a long double value that can have huge values.
At one time I have this number represented as NSString and I need to convert it to long double. I see that the only API I have is
[myString doubleValue];
I don't see a longDoubleValue.
Trying to convert this number using doubleValue...
long double x = (long double)[#"3765765765E933" doubleValue];
gives me inf and the number in question is a legit long double value, as these numbers can go up to 1.18973149535723176502E+4932.
How do I do that?

Answer 1

Perhaps create a category on NSString yourself
NSArray *array = [myString componentsSeparatedByString:#"E"];
long double mantis = (long double)[array[0] doubleValue];
long double exponent = (long double)[array[1] doubleValue];
return mantis * exponent;
There will possibly be a loss of data though
edit
It would seem that long double on iOS is the same size as double. Maybe you will need a custom class to hold such large numbers.

Answer 2

You could probably do:
long double s = strtold(myString.UTF8String, NULL);
but if sizeof(long double) is the same as sizeof(double) as mag_zbc says, you might still get Inf.
If you want to go the pow() route, there is powl() which takes and returns long doubles.

Answer 3

You can do this using the C library sscanf function. Here is a sample Objective-C wrapper:
long double stringToLongDouble(NSString *str)
{
long double result = 0.0L;
int ret = sscanf(str.UTF8String, "%Lg", &result);
if (ret != 1)
{
// Insert your own error handling here, using NSLog for demo
NSLog(#"stringToLongDouble: could not parse '%#' as long double", str);
return 0.0L;
}
return result;
}
The return from sscanf will be 1 if it succeeds. For possible error returns see the documentation (man 3 scanf in Terminal) and you need to decide how to handle these, the above example just does an NSLog.
Note: The size & precision of long double may vary by platform/OS version. The above has been tested with your value on El Capitan and iOS 10 (simulator only) using Xcode 8.
HTH

Answer 4

In fact the answer of mag_zbc is almost there. The last line is incorrect.
Considering that the string has exponent, the correct is:
- (long double)longDoubleValue {
NSArray *array = [string componentsSeparatedByString:#"E"];
long double mantis = (long double)[array[0] doubleValue];
long double exponent = (long double)[array[1] doubleValue];
long double multiplier = powl(10.0L, exponent);
return mantis * multiplier;
}